(a) Field
The subject matter disclosed generally relates to the field of methods and devices used for making measurements in the medical field. More specifically, it relates to methods and devices for the accurate measurement of fractional flow reserve.
(b) Background of the Invention
Pressure guidewires are 0.014″ guidewire comprising an embedded pressure transducer devoted to measure intra-coronary pressure. More specifically, these guidewires are used to measure the pressure distal to a lesion. By calculating the ratio between the measured pressure distal to the lesion and some point more proximal, most commonly in the ascending aorta or nearby the coronary oestium, the fractional flow reserve (FFR) is obtained. The FFR is now commonly used to assess the severity stenosis and thereby informs the physician as to the most appropriate treatment strategy. Recently there has been greater clinical acceptance of the importance of measuring translesional pressure and calculating FFR prior to deciding whether to place a stent. As detailed in Fearon et al. “Rationale and design of the fractional flow reserve versus angiography for multi-vessel evaluation (FAME) study” American Heart Journal (2007) vol. 154 (4) pp. 632-636, which is hereby incorporated by reference, FFR guided PCI therapy leads to better outcome than angiography guided PCI therapy, whereas stenosis with an FFR greater than 0.80 are not stented, while stenosis with an FFR lower or equal to 0.80 are stented.
Although there exists a gray zone with FFR measurements between 0.75 and 0.80 where other factors should be considered for the treatment strategy, the decision to stent or not is strongly influenced by the FFR value, where an error greater than 0.02 is considered as clinically relevant. In order to get an accurate FFR measurement, it is critical to have accurate distal and proximal pressure measurements. A variety of pitfalls exist, such as the positioning of the guide catheter and the height adjustment of the external aortic pressure transducer. However, a careful operator can easily eliminate those factors of error and routinely obtain accurate FFR measurements.
Accurate pressure measurements are obtained by equalizing the pressure guidewire with the aortic pressure. The method used by the systems available on the market involves bringing the pressure guidewire at the site of the aortic pressure, and adding an offset to the pressure guidewire such that both systems display the same level of pressure. Of importance here is the fact that this equalization follows the zeroing of both aortic and distal pressure sensors and hence, there shouldn't be any offset between the pressure sensors, but there should rather be a difference in their respective gain, or the sensitivity of the pressure guidewire (distal pressure sensor) is different from the sensitivity of the aortic pressure transducer (external transducer). The equalization of two pressure sensors, for which pressure zeroing was performed adequately, by way of adding an offset may lead to an error that can be clinically significant.
On the other hand, there are situations where equalizing the pressure by way of adjusting the gain may also lead to clinically significant error. Most of today's pressure guidewires are based on the use of a piezo-electric sensor embedded within the distal end of the guidewire. While from the above it may sound better to equalize those sensors by way of adjusting the gain, it may also lead to very significant error when equalizing a pressure guidewire for which the pressure error results from the re-connection of the guidewire to the interface cable, e.g., after using the pressure guidewire for the delivery of a stent. It is indeed known to those in the art that re-connecting such an electrical guidewire may lead to a pressure error caused by a change in the electrical contacts which is thereby associated to an offset rather than a difference in the gain.
There is therefore a need for a method for equalizing the pressure of a pressure device with the pressure of another pressure transducer in such a way that they both accurately measure a same pressure throughout the duration of a procedure.